drm/amdgpu: refine SR-IOV firmware VRAM reservation to protect data

The previous solution will create a zero buffer on the system
domain and then move the zeroes to the VRAM. This will break the
original data on the VRAM.

Refine the code to create bo on VRAM domain directly and then remove
and re-create mem node to the exact position before bo_pin. This can
avoid breaking the data and will not cause eviction.

Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: monk liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>